1. Field of the Invention
This invention relates to a multi-user communication system and in particular to a reduced algorithm receiver that limits the number of searches between consecutive symbol cycles using information from an encoder state diagram thereby reducing the amount of symbols being checked and signal processing requirements.
2. Description of Related Art
A co-channel, multi-user environment poses a severe signal processing challenge, as the number of users increases, the number of possible signal states that must be evaluated by the receiver rises exponentially. Despite dramatic improvements in processor speeds, the higher frequencies utilized in many communications systems require efficient real-time signal processing. Many systems employ convolutional encoding.
Prior art methods for multi-user detection match every possible combination of transmitted data from multiple users to determine the best-fit (most likely) match of the transmitted and received data. This match is an estimation of the actual transmission of each user. As the number of users increases, the processing requirements increase exponentially.
U.S. Pat. No. 5,790,606 issued Aug. 4, 1998 to Paul W. Dent and assigned to Ericsson Inc., of North Carolina discloses a type of modulation using multiple spatially distributed antennas and recombines that data of a receiver for post-processing. A modified Viterbi (maximum likelihood) decoder is used to decode the bits from the multiple users and multiple antennas. The operation of this system is questionable when bit transitions of the various co-channel transmitters are not aligned in time at every antenna (a virtual impossible condition to meet). Also, this system is very complex and can be very large depending on the space required for the multiple antennas.
U.S. Pat. No. 5,917,852 issued Jun. 29, 1999 to Lee A. Butterfield et al. and assigned to L-3 Communications Corporation of New York, N.Y. discloses a method and apparatus of using independently scrambled Code Division Multiple Access (CDMA) signals to reduce the multi-user interference between the different radios in the system. However, the scrambling occurs outside the actual error correction encoding process and adds additional complexity to the signal.
U.S. Pat. No. 6,122,269, issued to Wales on Sep. 19, 2000 performs multiuser detection and parameter estimation for a packet radio application. This procedure uses MUD to jointly demodulate packets which have unintentionally collided in time. The procedure uses known symbol sequences to solve for the unknown channel impulse response coefficients, and a correlation process to locate the positions of the known symbol sequences. In the case of short “snapshots” (vectors of received waveform samples), the correlation process will produce noisy data, and inaccurate known symbol sequence position estimates.
U.S. patent application Publication Ser. No. US 2002/0037061 A1, published Mar. 28, 2002, entitled “System For Parameter Estimation and Tracking of Interfering Digitally Modulated Signals”, filed Aug. 31, 2001, by Rachael L. Learned, application Ser. No. 09/943,770 and assigned to the present Assignee discloses a multiuser detection system in which interfering signals are purposely allowed to exist. A parameter estimation unit is provided which utilizes parallel processing for determining the channel for each received signal, which, in essence provides for each signal the received power, multipath structure, phase of the oscillator, timing offset relative to the base station clock and carrier frequency offset, with the system providing realtime uninterrupted estimates of these parameters required by the signal separation unit. However, this MUD system estimates the parameters by assuming that signals are added to the propagation channel one at a time.